1. Field of the Invention
The present invention relates to a pilot signal detection method and a receiver, or more particularly, to a pilot signal detection method suitably used for a communication scheme called a CDMA (code division multiple access) (hereinafter referred to as the CDMA scheme) and a receiver for detecting the pilot signal.
2. Description of the Related Art
In a communication system with a single base station shared by a plurality of mobile stations, various conventional communication schemes including the frequency division multiple access, the time division multiple access, the code division multiple access and so on have been used in order to avoid the interference of the channels of communication with the mobile stations. Each scheme has its advantages and disadvantages, and an actual communication scheme is selected in accordance with the object of the communication system involved.
In the code division multiple access (hereinafter called the CDMA) scheme, for example, the modulation wave of the same carrier frequency is spread over a wider band than the original frequency band (hereinafter referred to as the spectrum spread) using the PN (pseudo random noise sequence) code which is a specific code assigned to each channel while at the same time multiplexing and transmitting each modulation wave subjected to the spectrum spread. Also, the spectrum spread signal received is synchronized with the PN code supplied through the channel to be demodulated thereby to identify the desired channel.
Specifically, at the transmitting end, a different PN code is assigned to a different channel. The PN code is a pseudo random sequence code. Each modulation wave transmitted through each channel is multiplied by a different PN code and subjected to the spectrum spread at the transmitting end. Each modulation wave is modulated in a predetermined process before being subjected to the spectrum spread. In this way, each modulation wave subjected to the spectrum spread is multiplexed and transmitted.
At the receiving end, on the other hand, the receiving signal transmitted from the transmitting end is multiplied while being synchronized by the same PN code as assigned to the channel to be demodulated. In this way, only the modulation wave transmitted through an intended channel is demodulated.
As described above, in the CDMA scheme, once the same code is set at the transmitting and receiving ends, the direct communication can be established for each call. According to the CDMA scheme, the modulation wave is subjected to the spectrum spread using a different PN code for each channel. At the receiving end, therefore, only the spectrum spread signal transmitted through the channel to be demodulated can be demodulated. Further, this scheme has a high confidentiality due to the fact that the PN code is a pseudo random sequence.
In a mobile communication system using the CDMA scheme, the base station at the transmitting end repeatedly transmits a PN code (hereinafter called the pilot signal) for securing and maintaining the synchronization with a mobile station and reproducing the clock. The mobile station at the receiving end detects each pilot signal transmitted by a plurality of base stations and assigns the detected timing to the demodulators. The PN code is generated in the demodulator of the mobile station. The demodulator multiplies the PN signal by the spectrum spread signal transmitted from the intended base station at the assigned timing and thus demodulates the signal.
Specifically, in the mobile communication system using the CDMA scheme, each base station transmits the PN signal of a different timing as a pilot signal. The mobile station detects the timing of the pilot signal supplied from an intended base station, and synchronizes the PN code generated in the demodulator with this timing, thus correctly demodulating only the spectrum spread signal transmitted from the intended base station.
It has been described above that each base station transmits the PN code at a different timing. Nevertheless, the PN codes themselves have the same code pattern. In other words, the difference in timing of the pilot signal for each base station represents the difference between the PN codes as it is.
The mobile station may move while communicating with a base station. Therefore, the environment in which the received wave propagates changes every moment. Specifically, since obstacles such as buildings and so on exist between the base station and the mobile station and far mountains reflect the radio wave, the radio wave received by the mobile station is always a combination of a plurality of reflected waves. Further, the mobile station itself moves at different speeds. Therefore, the timing at which the pilot signal is detected at the mobile station changes every moment, and a phenomenon called the fading occurs, thereby making it sometimes impossible to detect the pilot signal. In order to continue detecting the pilot signal correctly in the fading environment, a radio telephone system using the CDMA communication scheme as the communication scheme is so configured that the first pilot signal received by the mobile station is set at the center of a pilot signal search window as a time reference.
Specifically, under the state that the pilot signal search window is set as shown in FIG. 1A, a detection timing T_ref of the first pilot signal in the window is set as a reference timing for determining the next pilot signal detection period. More specifically, as shown in FIG. 1B, the timing (referred to as the Old T_ref) after the lapse of a predetermined period of time from the first detection timing T_ref to the time point when the next pilot signal is transmitted is set at the center of the pilot signal search window, within which the pilot signals are detected. The detection timing T_ref of the first pilot signal in the window constitutes a new timing T_ref providing a reference for setting the next window.
In this process of detecting the pilot signals in the window set as described above, however, all the pilot signals transmitted do not enter the search window depending on the prevailing signal conditions. As shown in FIG. 1B, for example, some pilot signals may exist outside the window and fail to be detected. Especially, in the fading environment where the arrival timing of the pilot signals considerably changes or in an environment where the arrival timing of the pilot signals is considerably delayed, the probability of the failure to detect the pilot signals increases.
A method conceivable for preventing this inconvenience is to reduce the probability of failure to detect the pilot signal by setting a wider search window. Such a method, however, consumes a considerable period of time for detecting the pilot signals and the power required for the detection is undesirably increased.
Accordingly, an object of the present invention is to provide a pilot signal detection method and a receiver which can detect the pilot signals in simplistic and accurate fashion.
In order to achieve the object described above, according to the present invention, there is provided a pilot signal detection method for receiving a spectrum spread signal and detecting an arbitrary pilot signal from the spectrum spread signal thus received, including the steps of generating a search window of a predetermined time width for each substantially predetermined period of time and detecting pilot signals existing in the window thus generated, and setting the current window at a position corresponding to the distribution of the pilot signals detected in an immediately preceding window.
According to the invention, even in the case where the delay amount of a pilot signal transmitted is considerably large, the particular pilot signal having a large delay amount is highly liable to exist in the window set for each period.